In pursuance of the problem of the significance of our finding that the mitochondrial DNA in patients with myelogenous leukemia is of double-length, we are now examining the effect of feeding diethylstilbestrol to rabbits. This study was stimulated by the finding of others that double-length mitochondrial DNA occurs in the thyroids of beef cattle as a single exception to our earlier finding that the double-length molecules were diagnostic for myelogenous leukemia. We plan to extend our studies of the properties and mode of action of the nicking-closing enzyme which we have purified from mouse nuclei. This enzyme has been implicated in the replication and/or the condensation of both SV40 viral DNA and chromosomal DNA in eukaryotic cells. We are continuing our examination of in vitro constructed recombinants from mitochondrial DNA and a bacterial plasmid DNA. The mitochondrial DNA portions of the recombinant isolated from bacteria have lost alkali-sensitivity and appear to be otherwise faithful copies of the original eukaryotic DNA. BIBLIOGRAPHIC REFERENCES: Isolation and Partial Characterization of the Relaxation Protein From Nuclei of Cultured Mouse and Human Cells. Hans-Peter Vosberg, Lawrence I. Grossman, and Jerome Vinograd. European Journal of Biochemistry, (1975) In press. Isolation and Properties of a Nicking-Closing Protein from Mammalian Nuclei. Hans-Peter Vosberg and Jerome Vinograd. ICN-UCLA Symposium on DNA Replication and its Regulation (1975) In press.